Tandem electrode welder and method of welding with two electrodes

ABSTRACT

An electric arc welding apparatus comprising at least a first consumable electrode and a second consumable electrode movable in unison along a welding path between the edges of two adjacent, mutually grounded plates, a first power supply for passing a first welding current at a first low frequency between the first electrode and the two plates, a second power supply for passing a second welding current at a second low frequency between the second electrode and the two plates, where each of the power supplies includes a three phase voltage input operated at line frequency, a rectifier to convert the input voltage to a DC voltage link and a high frequency switching type inverter converting the DC voltage link to a high frequency AC current, an output rectifier circuit to provide a positive voltage tenninal and a negative voltage terminal, and an output switching network operated at a given low frequency for directing a pulsating welding current at the given low frequency from the two terminals across one of the electrodes and the plates, and a circuit for independently adjusting the given low frequency so the value of the first low frequency of the first power supply is different from the second low frequency of the second power supply.

The present invention relates to the art of electric arc welding andmore particularly to a welding apparatus using two consumable electrodesand the method of welding with tandem consumable electrodes.

INCORPORATION BY REFERENCE

The present invention utilizes a high frequency inverter of thetransistor switching type for converting a three phase input powersource to a load coupling transformer from which the AC output of theinverter may be rectified to create current flow between the electrodeand workpiece of a welding operation. Such welders employ a pulse widthmodulator operated at over 18 kHz for controlling the magnitude of thewelding current flowing through the welding operation. These invertersare well known in the art and are generally shown in Blankenship5,349,157 and Blankenship 5,351,175, which patents are incorporated byreference herein as background information. They illustrate a threephase inverter with current controlled by a high frequency pulse widthmodulator directing current pulses to the output transformer of theinverter. The three phase inverter has a pulse width modulator operatedby an error amplifier for controlling the current at the output of theinverter. Output switching networks in arc welders for creating ACwelding current from DC terminals are disclosed in Stava 4,861,965 andStava 4,947,021, also incorporated by reference herein. Shutt 4,246,463and Fratiello 5,155,330 show tandem mounted consumable electrodes usedfor welding the joint between two spaced plates. These patents are alsoincorporated by reference as background information, since they relateto the field to which the present invention is directed.

BACKGROUND OF INVENTION

The present invention relates to an electric arc welder of the typeusing two consumable electrodes for welding the joint between two edgesof relatively thick plates, which plates may be in the form of thecylindrical ends of two pipe sections being welded together in a pipewelding operation or, in practice, seam welding of a formed plate into apipe. In such welding procedures, two tandem mounted consumableelectrodes are moved in unison along the joint so two layers of moltenmetal are deposited first from the front electrode and then from thetrailing electrode to fill the joint between the two beveled edges ofthe adjacent edges of the plate forming a pipe. These edges arehereinafter referred to as adjacent plates for global application. Theinvention is applicable for seam welding of pipe; however, pipe weldingwill be described in a welding operation where the ends of the pipesections are welded together. The invention is much broader inapplication and may be used to butt weld two adjacent heavy plates, suchas the plates forming sections of gantries for oil rigs, armor of platesused in ship building or seam welding of pipe. Tandem consumableelectrodes deposit large amounts of molten metal and cause the metal tobe fused in the joint between the plates at high rates as necessary inwelding heavy plates. In the past, a single phase power supply wasnormally used to produce the welding current for both of the tandemelectrodes. The frequencies of the welding operation for the adjacentelectrodes were the same causing extreme arc generated interference.Such systems required elaborate connections, for instance a Scottconnection that produce an electrical phase shift. The frequencies ofthe welding procedures were dictated by the line frequency of the singlephase input power supply. Thus, the frequencies of the pulsating weldingcurrent for the tandem mounted electrodes was generally the same fixedvalue determined by the 50 Hz or 60 Hz input voltage. The prior artwelders using tandem consumable electrodes had generally caused anunbalance in the three phase power system and had welding frequenciescontrolled by the line frequency of the input voltage. This limitationwas extremely detrimental when the high currents of pipe welding wereused, which high currents exceed about 200 amperes and were often ashigh as about 1000-1200 amperes, or more. When tandem mounted electrodesare subject to relatively fixed low frequencies, determined by the linefrequency, and conduct extremely high currents, used in pipe welding andwelding thick plates, arc interference presents a serious problemrequiring complicated connections and shielding. In the prior devices,it was somewhat normal practice for each of the electrodes to be drivenby current having the same frequency, such as 50 Hz or 60 Hz. The onlyway to reduce arc interference was to shift the phase of the linecurrent being directed to each of the tandem electrodes. This procedurewas extremely complicated. The process was dictated by the linefrequency and the phase shifting did not fully alleviate arcinterference. Increasing the frequency of the welding current above100-200 Hz to reduce interference was not practical in the priorsystems. Thus, there is a substantial need for an improved electric arcwelder using two or more consumable electrodes which do not have theproblem of arc interference and do not utilize high current pulsefrequencies.

THE INVENTION

The present invention overcomes the problems experienced in efforts toprovide an electric arc welder that can effectively utilize twoconsumable electrodes, while reducing or generally eliminating arcinterference without requiring high frequencies for the welding current.Although the invention can be used in a DC mode, preferably it involvesan electric arc welder that directs AC welding currents to two separateconsumable electrodes, which welder can be driven by a single threephase power supply while the welding operation at each electrode isindependently controlled. The frequency of the welding current for eachof the two electrodes is independently controlled and does not dependupon the input line frequency. By using the present invention, a threephase input voltage is used for a tandem electrode welder. Thus, thereis a balanced input power, not an unbalanced single phase as in theprior art. The three phase voltage input has a line frequency of 50 Hzor 60 Hz; however, this frequency does not dictate the frequency of thewelding current at each consumable electrode. This welder develops highwelding current for heavy plates with the current exceeding about 200amperes and is normally at least about 1000 amperes. Although two tandemmounted consumable electrodes are used in the preferred embodiment, itis possible to use three or more electrodes in the electric arc welderof the present invention.

In accordance with the invention, the electric arc welder includes afirst and second consumable electrode, which electrodes are moved inunison along a welding path between the edges of the two adjacentmutually grounded plates, such as the seam in a pipe manufacturingoperation. There are two separate power supplies for passing weldingcurrents between the individual electrodes and the plates forming thegrounded workpiece of the welding operation. The power supplies each areconstructed to provide low frequency current pulses for the weldingoperation performed by the individual electrodes. In the preferredembodiment, the power supplies include a three phase voltage input,which input is operated at line frequency, such as 50 Hz or 60 Hz. Sincethe line frequency is isolated and does not control the output frequencyat the individual electrodes, the same three phase voltage source can beused for both power supplies separately controlling the tandem mountedelectrodes. The three phase power supply is rectified to convert theinput voltage to a DC voltage link and a high frequency switching typeinverter converts the DC voltage link to a high frequency AC current.The high frequency switching type inverter is controlled by a pulsewidth modulator operated at a frequency generally greater than 20 kHzwith a duty cycle that is adjusted to control the magnitude of theoutput current at the electrodes. The high frequency AC current formingthe output of the high frequency switching type inverter is directedthrough an output or load transformer having a secondary winding fordriving an output rectifier circuit to provide a positive voltageterminal and a negative voltage terminal. As so far described, each ofthe individual power supplies for each of the two consumable electrodesis driven by the same three phase source, but creates individual outputsin the form of a positive terminal and a negative terminal. In thespecific power supply used in the present invention, the outputterminals provide power to a switching network driven at a given lowfrequency for directing the polarity of the welding current from theterminals of the power supply across one of the electrodes and the twospaced plates forming the workpiece of the welding operation. By usingan independently controlled output switching network at the output sideof a standard high frequency inverter, the frequency of the weldingcurrent, if AC or pulsating DC, is independently controlled by theoutput switching network so the power supply components before theswitching network produce a DC current. The frequency at the output ofthe power supply that controls the frequency of the current in eachelectrode is independently controllable and not dependent upon eitherthe input frequency of the power supply or the frequency of the weldingcurrent used by the other electrode. This operational isolation freesthe tandem electrode electric arc welder to allow individual control ofthe welding current frequency for each of the separate and distinctconsumable electrodes. By this architecture for the electric arc welder,the low frequencies at the two electrodes can be controlled to less than300 Hz and preferably in the general range of 5-200 Hz. The invention isalso operative to provide DC current, either continuous or pulsing inboth polarities. Three phase input voltage is used, thus allowingbalanced operation of the welder without dictating the frequency of thewelding currents.

In accordance with another aspect of the present invention there isprovided an electric arc welding apparatus comprising at least a firstconsumable electrode and a second consumable electrode movable in unisonalong a welding path between the edges of two adjacent, mutuallygrounded plates. A first power supply is provided for passing a firstwelding current at between the first electrode and the plates and asecond power supply is provided for passing a second welding currentbetween the second electrode and the plates. Each said power supplyincludes a three phase voltage input operated at line frequency, arectifier to convert the input voltage to a DC voltage link and a highfrequency switching type inverter converting the DC voltage link to ahigh frequency AC current, an output rectifier circuit to provide apositive voltage terminal and a negative voltage terminal, and an outputswitching network operated for directing welding current from theterminals across one of the electrodes and the plates, and a circuit forindependently adjusting the output switching networks so the value ofthe first welding current of the first power supply is different fromthe second welding current of the second power supply. The switchingnetworks can maintain a continuous DC current of either polarity or anAC current at a low frequency by alternating the switches of thenetwork.

The individual power supplies include circuits for setting the lowfrequency used by each of the separate electrodes. In accordance with anaspect of the invention, at least one of the power supplies includesmeans for causing the low frequency of that power supply to vary as afunction of time. By using this concept, one of the electrodes isoperated at a fixed frequency below about 200 Hz, while the otherelectrode is operated at a frequency that continues to vary between twoextremes, such as 10 Hz to 300 Hz. By varying the low frequency of onepower supply between two extremes, while the low frequency of the otherpower supply is fixed, there is only one frequency at which the twoelectrodes have identical frequencies. In accordance with another aspectof the present invention, the low frequencies of both electrodes arevaried as a function of time, but not in synchronization. Thus, thefrequency of the welding current at one electrode sweeps between 10 and300 Hz. At the same time, the welding current of the other tandemmounted electrode sweeps between 10 and 300 Hz. This sweeping of theelectrode welding frequency prevents arc interference and does notrequire complicated connections. In accordance with still another aspectof the present invention, the inverter between the three phase inputvoltage and the low frequency welding current is operated at anextremely high frequency, i.e. at least about 20 kHz. This is standardoperation for a high frequency switching inverter. The invention usesthe inverter stage to isolate the electrically separate input power fromthe welding parameters for each tandem mounted electrode. The outputwelding current is pulsating. It can be either low frequency DC pulsesor low frequency AC pulses or continuous DC current of either polarity.Indeed, in one example, the low frequency pulses at one electrode are DCpulses, whereas the low frequency pulses at the other electrode are ACpulses. The frequency of the pulses is varied progressively or sweepbetween 50 and 200 Hz so there is no arc interference caused by beatingof the frequencies used in the closely positioned consumable electrodes.

A variety of architectures can be employed for creating the individualpower supplies. In accordance with the preferred embodiment, each powersupply includes an inverter for converting the AC three phase voltage toa DC current source output having a maximum current of at least 200amperes with a positive output terminal and a negative output terminalto provide DC energy to an output switching network. The network, in thepreferred embodiment of the present invention, includes a firsttransistor based switch in series with the positive terminal of thepower supply, a series inductor segment, one of the electrodes and thespaced plates or workpiece. By closing this series circuit, a pulse of afirst polarity is directed to the welding operation. The network alsoincludes a second transistor based switch in series with the negativeterminal, a second inductor segment, the same electrode and the plates.Such switching networks are shown in Stava 4,861,965 and Stava4,947,021. Control means ultimately turn the first switch ON and thesecond switch OFF at a first switch reversing point and turn the secondswitch ON and the first switch OFF at a second switch reversing point.Consequently, AC high welding current is created with alternate positiveand negative current pulses. This switch network creates the lowfrequency welding current for one of the electrodes. The samearchitecture is used to create the low frequency welding current of theother electrode. By merely adjusting the control means of the two powersupplies, the frequency of the welding current is adjusted. Otherelectrical architectures can be used for the power supplies so long asthey have an input voltage operated at line frequency, an inverter toconvert the input voltage to a DC voltage link and a high frequencyswitching type inverter for converting the DC voltage link to a highfrequency AC current that is rectified by a circuit to provide apositive voltage output terminal and a negative voltage output terminal.These terminals are used for directing DC energy to the switchingnetwork. Thus, the power supply is driven by a three phase voltagesource at a line frequency, but creates the desired low frequency outputcurrent. The ability to control the low frequencies of the outputcurrents independent of each other results in an improved tandemelectrode electric arc welder.

Another aspect of the present invention involves use of a first andsecond power supply for driving first and second consumable electrodesmoved in unison in the joint between two plates. Each of the powersupplies have switch networks for creating low frequency weldingcurrents. By generating a synchronizing signal with a given frequencydetermined by the rate of synchronizing commands, the power supplies areforced to a given polarity when a synchronizing command, such as a logic1 or a positive going leading edge, is presented to the switchingnetwork of the power supply. By delaying the synchronizing signal at oneof the power supplies, the low frequency of that power supply is offsetor phase shifted. Consequently, by use of a synchronizing signal and adelay circuit, the low frequencies of the welding currents are out ofphase. Using a synchronizing signal, the alternating weld current isforced to a given polarity at the time of a command in the signal. Inone embodiment, the synchronizing signal is a positive command and anegative command. When the switching circuit receives a positivecommand, the weld current is forced to positive polarity. The negativecommand forces the weld current to a negative polarity. Since thesynchronizing signal is delayed at one power supply, the forcing actionof the command makes low frequency welding currents which are out ofphase. This is done without a Scott connection. As another aspect, thesynchronizing command starts a weld current oscillating at a givenfrequency created at the power supply. This given frequency producesalternations between synchronizing commands or between a positivecommand or a negative command.

The primary object of the present invention is the provision of animproved tandem electrode electric arc welder, which welder can be usedfor seam welding pipe from a rolled plate and similar welding operationsemploying pulsating high welding currents without arc interference.

Yet another object of the present invention is the provision of anelectric arc welder, as defined above, which welder includes separatelydriven consumable electrodes, each of which is driven by a high weldingcurrent having a low frequency with the frequencies being adjusted orvaried on a time basis or is operated at a given DC polarity.

Still a further object of the present invention is the provision of anelectric arc welder, as defined above, which electric arc welderutilizes a three phase input voltage allowing balancing of the inputpower for the welding operation. Such input power balancing is extremelyimportant when using extremely high currents, as needed for welding theseam of a pipe.

Still a further object of the present invention is the provision of animproved electric arc welder, as defined above, which electric arcwelder can use tandem consumable electrodes without unbalancing thepower source or creating undue arc interference.

These and other objects and advantages will become apparent from thefollowing description taken together with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a pictorial view schematically illustrating a tandemconsumable electrode type welder;

FIG. 2 is a wiring diagram showing the two power supplies for providingpulsating welding current for each of two tandem mounted consumableelectrodes;

FIG. 3 is a wiring diagram schematically illustrating the controlcircuit for independently adjusting the low frequencies for the tandemelectrodes of the preferred embodiment of the present invention;

FIG. 4 is a view comprising two pulse curves above a schematicillustration of tandem electrodes using the currents of pulse curves;

FIG. 5 is a wiring diagram showing a modification of the control circuitshown in FIG. 4;

FIG. 6 is a view similar to FIG. 4 showing the operating characteristicsof the control circuit shown in FIG. 5;

FIG. 7 is a wiring diagram showing still a further modification of thecontrol circuit for independently controlling the low frequency of thecurrent pulses at the two electrodes;

FIG. 8 is a view similar to FIG. 4 showing the operating characteristicsof the control circuit of FIG. 7;

FIG. 9 is a wiring and block diagram of a system constituting amodification of the present invention using a synchronizing signal and adelay circuit to give a phase shift; and,

FIG. 10 is a wiring and block diagram of a subroutine modifying aportion of the system shown in FIG. 9.

PREFERRED EMBODIMENT

Referring now to the drawings wherein the showings are for the purposeof illustrating the preferred embodiment of the invention only, FIG. 1shows a welder 10 including tandem mounted consumable electrodes 12, 14in the form of welding wires advancing from supply reels WF1, WF2,respectively. Tandem electrodes 12, 14 are pushed through contactholders 16, 18 forming electrical contacts that provide pulsating, lowfrequency welding current between electrodes 12, 14 and spaced heavyplates 20, 22. These plates are the workpiece WP of the weldingoperation. Electrodes 12, 14 are moved in unison by schematicallyillustrated frame F to travel along a path defined by joint J betweenthe spaced edges of plates 20, 22 to form a weld bead 30 overlaid bybead 32. The plates are joined at beads 30, 32. In the preferredembodiment, plates 20, 22 are the adjacent edges of a cylindrical pipeto weld the edges into a pipe. Frame F is transported by a tractorriding on a track to move electrodes 12, 14 along joint J. Metal isdeposited in the joint between the edges of the rolled plate to weld theplate together when cylindrical pipe is being manufactured. FIG. 1 showspower supply PS which is powered by a three phase input voltage sourcefrom input lines L1, L2 and L3. The voltage input lines provide theenergy used to create the low frequency welding current in accordancewith the invention. By employing tandem electrodes 12, 14 a large amountof metal is deposited in beads 30, 32 between plates 20, 22 as tractor Fmoves along the plates. In accordance with the invention, power supplyPS is divided into two separate and distinct individual power suppliesPS1, PS2 to provide power to the individual tandem mounted electrodes12, 14, respectively. Power supply PS is shown in detail in FIG. 2wherein first power supply PS1 is shown in detail. Second power supplyPS2 is a duplicate of power supply PS1 and is thus illustrated onlygraphically. The description of first power supply PS1 applies equallyto second power supply PS2.

Referring now to dedicated power supply PS1. This power supply, in thepreferred embodiment, includes an inverter stage 40 including a highfrequency switching type inverter 42 provided with power from threephase voltage source 44 having a frequency of 50 or 60 Hz according tothe local line frequency. The AC input voltage is rectified by rectifier46 to provide a DC link 48 directed to the input of inverter 42, whichis operated at a high frequency in excess of 18 kHz. The output or loadof inverter 42 is transformer 50 having primary winding 52 and secondarywinding 54 with a grounded center tap 56 connected to ground 58. Highfrequency pulses at the input of transformer 50 are drastically higherthan line frequency at voltage input 44 to reduce the size of thecomponents necessary for the inverter. Secondary winding 54 is directedto rectifier circuit 60 having diodes D1, D2, D3 and D4 to create apositive output terminal 62 and a negative output terminal 64 connectedto the output switching network 70. The switching network operates at alow frequency of less than 200-300 Hz. Output switching network 70includes two transistor type switches SW1 and SW2, usually in the formof IGBT's that can be turned on and off according to the logic on baselines 116, 118. To dissipate high voltages when switches SW1, SW2 areoff, snubber networks 100, 102 are connected across the switches.Network 70 is used for pulsating high welding currents substantiallyover 200 amperes. A single output inductor 110 is divided into positivepulse section 112 and negative pulse section 114. In this manner, an ACcurrent is created in output lines 120, 122 connected to electrode 12and grounded plates 20, 22. Power supply PS2, shown in the bottomportion of FIG. 2, has output line 120 connected to tandem electrode 14and output line 122 also connected to the grounded base plates 20, 22.By alternating the logic on base control lines 116, 118 in succession,an alternating current is applied to the welding circuit of theindividual tandem mounted electrodes. Inverter 42 is controlled bymicroprocessor controller 200, which controller is somewhat standard. Ithas an output to control pulse width modulator 202, driven by oscillator204. The oscillator in practice has a frequency exceeding 18 kHz andpreferably has a frequency in the range of 20-40 kHz. Consequently, theswitching inverter 42 operates at a high frequency in excess of 20 kHzto convert the three phase input voltage at source 44 into a highfrequency current output at primary winding 52 of transformer 50. Thepulse width modulator is operated at the frequency of oscillator 204 bypulses on control line 206. The duty cycle of the individual pulsescontrols the amount of current being created by inverter 42 and isdetermined by the voltage on input line 210 which is the output of erroramplifier 212 generating a voltage according to the difference between avoltage representing actual arc current from shunt 220 by way of line222. Feedback circuit 224 applies a voltage on input 226 that representsthe instantaneous arc current or voltage. A second input 230 is the waveshaping output signal from controller 200. The relationship betweeninput 226 and input 230 at error amplifier 212 determines the voltage online 210 and, thus, the duty cycle at any given time for pulse widthmodulator 202. This control circuit is standard architecture for aswitching type inverter so the current being transmitted by inverter 42is controlled in accordance with the output signal on line 230 ofcontroller 200.

By using the power supplies PS1 and PS2 as shown in FIG. 2, both ofwhich are driven by input voltage source 44, a controlled low frequencyalternating current is created at each electrode 12, 14. The lowfrequency is determined by the frequency at which the logic alternateson base control lines 116, 118. The logic on these lines is generated bya software program or subroutine processed by a microprocessor incontroller 200. This program is schematically illustrated as a hardwiredcircuit in FIG. 3. As an additional operation output switching network70 of each power supply can be operated as a DC welder by closing eitherswitch SW1 or SW2 while the other switch is opened. In this DC operationthe current is controlled by the voltage on input line 230. Indeed, thevoltage on line 230 can be controlled to produce a selected DC waveslope through the closed switch SW1 or SW2.

Referring now to FIG. 3, the control for power supplies PS1 and PS2 areillustrated as hardwired flip-flops 240 and 240 a, respectively. Detailsof flip-flop 240 will be described. This description applies equally toflip-flop 240 a, wherein the elements have the same number, but aredesignated with the subscript a. Flip-flop 240 is a software program toproduce alternating logic in base control lines 116, 118 at a desiredfrequency. Flip-flop 240 controls the alternate switching of switchesSW1 and SW2 at a low frequency to produce a low frequency output fornetwork 70 as shown in FIG. 2. The logic in lines 116, 118 is the outputof non-coincident terminals 242, 244 of flip-flop 240. The logic onthese terminals is alternated according to the logic at either the setterminal 246 or the reset terminal 248. To change the logic on lines116, 118 the logic on terminals 246, 248 are reversed at a frequencydetermined by the control circuit 250 in the form of a software voltagefrequency oscillator 252 having an output frequency determined by thesetting or adjusted voltage of control circuit 254 for frequency f₁.This is the low frequency for alternating the logic on lines 116, 118.This is the frequency of the welding current at electrode 12. The outputof the voltage control oscillator 252 is the logic on line 256 connectedto set terminal 246 and through inverter 258 to reset terminal 248. Apositive pulse at the output of oscillator 252 sets flip-flop 240 tocreate a logic 1 in line 116. A logic 0 at the output of oscillator 252has the reverse effect and creates a logic 1 at terminal 248 and, thus,a logic 1 at the inverted output terminal 244 to produce a logic 1 inline 118. A logic 1 in line 116 or line 118 turns on the switch SW1 orSW2. When the logic 1 shifts to the opposite output line, the transistortype switch shifting to a base at logic 0 immediately turns off. Thus,by adjusting the output of circuit 254, frequency f₁ of network 70 inPS1 is determined. In a like manner, adjusting the frequency by changingcircuit 254 a produces a desired frequency f₂ for consumable electrode14. In accordance with the invention, a software program or otherstandard electrical architecture is used to control the frequency f₁ andf₂ of the current of the welding operation for electrode 12 andelectrode 14. These frequencies are controlled separately to prevent arcinterference. The operation of the invention as illustrated in FIG. 3 isshown in FIG. 4 wherein the frequency f₁ of electrode 12 is a lowfrequency, but substantially greater than the frequency f₂ for electrode14. Frequencies f₁ and f₂ have no relationship to the input linefrequency of voltage source 44 and no relationship to the high frequencyof inverter 42. Consequently, the present invention involves a tandemelectrode welder wherein the output network creates a given lowfrequency for each of the tandem electrodes. The frequencies areindependently controlled and have no relationship with each other. Thiswelder is an advance in the art and is advantageous in high currentwelding which arc noise must be suppressed.

In the preferred operation, a low frequency AC welding current iscreated at both electrodes. However, a DC current is used in analternative embodiment. Such embodiment includes single pole softwareswitch 260 at the output of oscillator 252 and a double pole switch 262connecting ground 264 to either terminal 270, terminal 272 or neutralterminal 274. Pull up resistor 266 is operative when switch 260 isopened. With switch 260 opened and switch 262 in the illustratedposition grounding the set terminal S of flip-flop 240, switch SW1 isheld closed and switch SW2 is held open. This provides a DC weldingcurrent with a position polarity. By shifting software switch 262 toground terminal R of flip-flop 240, a negative welding current isestablished. During the DC welding operation, the voltage on line 230can be controlled to give any wave shape such as pulses. These pulsesare at a selected frequency but are no AC in operation. With switch 260closed and switch 262 at neutral terminal 274, the preferred ACoperation is implemented. The second power supply has the samealternative operation by including software switches 260 a and 262 a.

The embodiment of the invention illustrated in FIG. 3 allows theoperator or welding engineer to adjust the individual frequency forelectrodes 12 and 14 or apply a DC welding current across theseelectrodes, which electrodes are moved in unison along the pathdetermined by joint J as shown in FIG. 1. By using power supply PS, asshown in FIG. 2, the frequency for the welding operation of eachelectrode is independently adjusted when the power supply PS is set forits preferred AC operation with switches 260 and 260 a closed andswitches 262 and 262 a in the neutral position. This invention isfurther modified as illustrated in FIGS. 5 and 6. A software networkillustrated as hardwired flip-flop 300 is used for controlling the logicon lines 116 c, 118 c to determine the frequency of the AC weldingcurrent at electrode 12. In this embodiment of the invention, softwarefor controlling the frequency of the AC current at electrode 14 is shownas a hardwired flip-flop 300 a. Flip-flop 300 and 300 a are identical sothe description of one flip-flop applies to the other, except flip-flop300 a controls the logic on lines 116 d and 118 d that are used tocontrol the switching rate of switches SW1, SW2 in network 70 of powersupply PS2 shown in FIG. 2. This controls the frequency of the weldingcurrent at electrode 14. Flip-flop 300 has output terminals 302, 304which are non-coincident to control the logic on lines 116 c, 118 c. Setterminal 306 and reset terminal 308 are used to change the logic atterminals 302, 304, in accordance with standard flip-flop technology.Circuit 310 is used to control the alternating logic applied toterminals 306, 308 and includes a voltage controlled oscillator 312 forcontrolling frequency f₃ in accordance with voltage from circuit 314.Circuit 314 sweeps the voltage on oscillator 312 between two extremesillustrated in FIG. 6 as X and Y representing a frequency in the generalrange of X=50 Hz and Y=200 Hz. Consequently, circuit 314 continuouslyadjusts the voltage at the input of oscillator 312 to change frequencyf₃ continuously and alternately between two extremes both substantiallywithin the low frequency region below about 200-300 Hz. In a likemanner, control circuit 314 a sweeps the frequency f₄ between twoextremes, which in practice are the same extremes as used for frequencyf₃. By sweeping frequencies f₃ and f₄ by circuits 314, 314 a, which arenot synchronized, there is random, continuous changing of frequencies f₃and f₄ between two low frequency extremes. The frequencies are neverfixed and have no relationship to each other. This operatingcharacteristic is schematically illustrated in FIG. 6 where frequency f₃for the welding operation of electrode 12 and frequency f₄ for thewelding operation of electrode 14 randomly sweeps back and forth betweenthe value X and the value Y so that there is no arc interference createdby the instantaneous cross summation of electromagnetic waves. Circuits314 and 314 a are software programs that change frequency randomly;however, hardwired circuits are also available for this purpose. In thepreferred embodiment as now implemented, the change in frequency thatsweeps between two levels is done internally in the software implementedby the microprocessor of controller 200.

In accordance with another embodiment of the present invention whenoperated in the preferred AC mode, the frequency of one of the tandemelectrodes is adjusted to a desired low frequency value and the otherelectrode is provided with a sweeping frequency. This is a combinationof the concepts used in the embodiment of the invention shown in FIG. 3and the embodiment shown in FIG. 5. This embodiment of the invention isshown in FIGS. 7 and 8 wherein frequency f₅ is adjusted to a fixed valuefor the switching frequency on output lines 116 e and 118 e. At the sametime, frequency f₆ representing the frequency of output control lines116 f and 118 f is variable between the low frequency extremes, such as50 Hz and 200 Hz. To accomplish the objective of this embodiment of theinvention, the software program in controller 200 incorporates a controlcircuit schematically illustrated as hardwired flip-flop 350 havingoutput terminals 352, 354 controlling output lines 116 e, 118 e,respectively, a set input terminal 356 and a reset input terminal 358.Circuit 360 controls the frequency of the alternating logic on terminals356 and 358 and includes a voltage controlled oscillator 362 with afrequency dictated by the adjusted voltage from control circuit 364.Output 366 is a pulsed signal for controlling the logic at terminal 356and the reverse logic through inverter 368 at terminal 358. Oscillator362 controls the frequency of the switching signals on output lines 116e, 118 e. At the same time, the frequency f₆, which is the frequency forelectrode 14, is controlled by a software program schematicallyillustrated as a hardwired flip-flop 370 having output terminals 372,374, a set terminal 376 and a reset terminal 378. Circuit 380 includes avoltage to frequency oscillator 382 controlled by the variable voltagefrom circuit 384 to cause a sweep in frequency of the logic at output386 connected to terminal 376 and of the inverted logic at terminal 378.The logic on output 386 is reversed by inverter 388. Consequently,electrode 12 is operated at an adjusted fixed frequency, whereas thetandem electrode 14 has a frequency that sweeps between X and Y, whichin practice is 50-200 Hz. This operation is shown in FIG. 8.

The invention is also applicable to a pulsating welding current, whichdoes not change polarities and is thus a DC current. This DC operationis implemented by software switches shown in FIG. 3. The DC pulsatingcurrent can create a series of current pulses through electrode 12 andthrough electrode 14 by a pulsating signal in line 230 during DCoperation. The frequency of the DC current pulses is a low frequency andis adjusted as so far described with respect to the preferred ACoperation of the invention.

Although the sweep circuits used in the AC implementation of theinvention can gradually change between voltage X and Y, software canrandomly select frequencies. There is no need to actually systematicallysweep between various extremes of the voltage. The invention, either ACoperation or pulsating DC operation, relates to the concept of producingfrequencies that are random and are not determined by the input linefrequency. The frequencies do not coincide, except occasionally.Consequently, without complicated circuitry, the arc interference isdrastically reduced. Continuous DC operation with a wave shapecontrolled by the voltage on line 230 is an alternative using powersupplies PS1 and PS2. Even though the description involves hardwiredcircuits and block diagrams, in practice these operations are performedby software programs using standard technologies.

To provide a simplified phase shift implementation of the presentinvention, system 301 illustrated in FIG. 9 has been devised. Electrode12 and 14 conduct alternating welding current from power supplies 311,313, respectively. Power supply 311 include high frequency inverterPS_(A) with a three phase input 320 and output terminals 322, 324. Apulse width modulator 330 causes the shape of the current pulses to becontrolled by error amplifier 332 with a pulse shape input 334 frommaster controller MC. Arc current sensed from shunt 340 as voltagesignal I_(al) is forced to follow the shape of the voltage signal frommaster controller MC on line 334. Switching network 351 is like network70 shown in FIG. 2. The network alternates as the logic on lines 353,355 is alternated by flip-flop A. The frequency of the welding currentat electrode 12 is controlled by the frequency of changes insynchronizing line 361 through non-inverted input 363 and the invertedinput 364 at terminals S, R, respectively. In a like manner, thefrequency of the weld current at electrode 14 is controlled by thenon-inverted input 400 and inverted input 402 of power supply 313. Othercomponents of this second power supply are the same as the likecomponents of power supply 311 and are numbered accordingly, except forpulse shape input 404 from master controller MC. The signal in line 361is delayed slightly by delay circuit 410 to produce a phase shiftbetween the AC welding currents of electrodes 12, 14. The delay issubstantial less than ½ period of the frequency on synchronization line361. If 60 Hz is used, delay is less than 5-6 ms.

In operation, system 300 has an AC synchronizing signal in line 361 withalternating positive commands and negative commands. In the illustratedembodiment, a positive command is a logic 1. The negative command is alogic 0. When the synchronizing signal on line 361 is a logic 1, apositive current pulse is caused by a logic 1 on line 353 and a logic 0on line 355. When this positive command is created, delay circuit 410delays a logic 1 on line 353 to flip-flop B. Thus, the weld current areout of phase by the delay of circuit 410. The low frequency of less thanabout 300 Hz is the same for both electrode 12, 14. Consequently, asimplified phase shift is provided using power supplies with the outputswitching networks shown in FIG. 2. If there are more than twoelectrodes, a further delay circuit 420 or up to N delays represent bydelay 422 can be used.

In accordance with the invention, the positive going command insynchronization line 361 can force the power supplies into a positivepulse. Thereafter this weld current will oscillate until a negativecommand forces the pulse into a negative pulse. By this concept used inan implementation of the invention, the frequency of the weld currentsis higher than the frequency of the synchronization signal whilemaintaining a forced phase shift. Such a system is shown in FIG. 10where the output of delay circuit 410 is a delayed synchronizationsignal. When the delayed signal shifts positive, i.e. to a logic 1 inline 412, pulse generator 450 starts operation at frequency f₁₀ with aforced first positive pulse. The next logic 1 in line 412 restarts thepulse generator with a forced positive pulse. These forced positivepulses are out of phase with similar forced pulses at flip-flop A by thedelay of circuit 410. Consequently, the modification of system 311 asshown in FIG. 10 gives out of phase welding currents with frequenciesdetermined at the individual power supplies, which frequencies aredifferent. In the system of FIG. 10 without a pulse generator at theinput of flip-flop A, flip-flop B is out of phase and frequency f₁₀ atflip-flop B is greater than the frequency of the synchronization signaloperating flip-flop A.

Master controller MC is shown by the dashed lines as a component of thefirst power supply and associated circuits. The second power supply isdriven from the controller associated with the first power supply. Inanother architecture, a master controller is a separate unit for drivingall power supplies.

The systems are implemented by software in computers controlling thewelders in accordance with standard practice. Combinations of softwareand hardware have also been used. The circuits can be modified toaccomplish the concept of a synchronizing signal used to offset thewelding currents and the low frequencies can be changed. To confine themagnitude of current during positive and negative pulse, mastercontroller MC give magnitude voltage levels to error amplifiers 332.Indeed, the voltage on lines 334 and 404 can control magnitude and pulseshape in both polarities.

Having thus defined the invention, the following is claimed:
 1. Anelectric arc welding apparatus comprising at least a first consumableelectrode and a second consumable electrode movable in unison along awelding path between the edges of two adjacent, mutually groundedplates, a first power supply for passing a first welding current at afirst low frequency between said first electrode and said plates, asecond power supply for passing a second welding current at a second lowfrequency between said second electrode and said plates, each of saidpower supplies including a three phase voltage input operated at linefrequency, a rectifier to convert said input voltage to a DC voltagelink and a high frequency switching type inverter converting said DCvoltage link to a high frequency AC current, an output rectifier circuitto provide a positive voltage terminal and a negative voltage terminal,and an output switching network operated at a given low frequency fordirecting a pulsating welding current at said given low frequency fromsaid terminals across one of said electrodes and said plates, and acircuit for independently adjusting said given low frequency so thevalue of said first low frequency of said first power supply isdifferent from said second low frequency of said second power supply. 2.An electric arc welding apparatus as defined in claim 1 wherein saidfirst and second low frequencies are in the general range of 5 to 200Hz.
 3. An electric arc welding apparatus as defined in claim 2 whereinsaid consumable electrodes are advance welding wires.
 4. An electric arcwelding apparatus as defined in claim 2 wherein said three phase voltageinput for each of said first and second power supplies is the same powersource.
 5. An electric arc welding apparatus as defined in claim 2wherein said means for adjusting said given low frequency of at leastone of said first and second power supplies includes means for causingsaid given low frequency to vary as a function of time.
 6. An electricarc welding apparatus as defined in claim 5 wherein said three phasevoltage input for each of said first and second power supplies is thesame power source.
 7. An electric arc welding apparatus as defined inclaim 2 wherein both of said first and second power supplies includemeans for causing said given low frequency to vary as a function oftime.
 8. An electric arc welding apparatus as defined in claim 7 whereinsaid three phase voltage input for each of said first and second powersupplies is the same power source.
 9. An electric arc welding apparatusas defined in claim 7 wherein said high frequency of each power supplyinverter is over about 20 kHz.
 10. An electric arc welding apparatus asdefined in claim 9 wherein said three phase voltage input for each ofsaid first and second power supplies is the same power source.
 11. Anelectric arc welding apparatus as defined in claim 9 wherein said firstand second low frequencies are in the general range of 5 to 200 Hz. 12.An electric arc welding apparatus as defined in claim 7 wherein saidpulsating currents of said power supplies are AC currents.
 13. Anelectric arc welding apparatus as defined in claim 12 wherein said threephase voltage input for each of said first and second power supplies isthe same power source.
 14. An electric arc welding apparatus as definedin claim 2 wherein said high frequency of each power supply inverter isover about 20 kHz.
 15. An electric arc welding apparatus as defined inclaim 2 wherein said pulsating welding currents of said power suppliesare DC currents.
 16. An electric arc welding apparatus as defined inclaim 15 wherein said three phase voltage input for each of said firstand second power supplies is the same power source.
 17. An electric arcwelding apparatus as defined in claim 2 wherein said pulsating currentsof said power supplies are AC currents.
 18. An electric arc weldingapparatus as defined in claim 2 wherein each said power suppliesincludes a pulse width modulator for controlling the voltage betweensaid terminals as a function of time to adjust the welding current. 19.An electric arc welding apparatus as defined in claim 1 wherein saidconsumable electrodes are advancing welding wires.
 20. An electric arcwelding apparatus as defined in claim 1 wherein said three phase voltageinput for each of said first and second power supplies is the same powersource.
 21. An electric arc welding apparatus as defined in claim 1wherein said means for adjusting said given low frequency of at leastone of said first and second power supplies includes means for causingsaid given low frequency to vary as a function of time.
 22. An electricarc welding apparatus as defined in claim 21 wherein said three phasevoltage input for each of said first and second power supplies is thesame power source.
 23. An electric arc welding apparatus as defined inclaim 21 wherein said high frequency of each power supply inverter isover about 20 kHz.
 24. An electric arc welding apparatus as defined inclaim 23 wherein said first and second low frequencies are in thegeneral range of 5 to 200 Hz.
 25. An electric arc welding apparatus asdefined in claim 24 wherein said three phase voltage input for each ofsaid first and second power supplies is the same power source.
 26. Anelectric arc welding apparatus as defined in claim 23 wherein said threephase voltage input for each of said first and second power supplies isthe same power source.
 27. An electric arc welding apparatus as definedin claim 21 wherein said pulsating currents of said power supplies areAC currents.
 28. An electric arc welding apparatus as defined in claim27 wherein said three phase voltage input for each of said first andsecond power supplies is the same power source.
 29. An electric arcwelding apparatus as defined in claim 21 wherein each said powersupplies includes a pulse width modulator for controlling the voltagebetween said terminals as a function of time to adjust the weldingcurrent.
 30. An electric arc welding apparatus as defined in claim 1wherein both of said first and second power supplies include means forcausing said given low frequency to vary as a function of time.
 31. Anelectric arc welding apparatus as defined in claim 30 wherein said threephase voltage input for each of said first and second power supplies isthe same power source.
 32. An electric arc welding apparatus as definedin claim 30 wherein said pulsating currents of said power supplies areAC currents.
 33. An electric arc welding apparatus as defined in claim 1wherein said high frequency of each power supply inverter is over about20 kHz.
 34. An electric arc welding apparatus as defined in claim 1wherein said pulsating welding currents of said power supplies are DCcurrents.
 35. An electric arc welding apparatus as defined in claim 34wherein said three phase voltage input for each of said first and secondpower supplies is the same power source.
 36. An electric arc weldingapparatus as defined in claim 1 wherein said pulsating currents of saidpower supplies are AC currents.
 37. An electric arc welding apparatus asdefined in claim 1 wherein each said power supplies includes a pulsewidth modulator for controlling the voltage between said terminals as afunction of time to adjust the welding current.
 38. An electric arcwelding apparatus as defined in claim 1 wherein said plates are the endsof two adjacent pipe sections.
 39. An electric arc welding apparatuscomprising at least a first consumable electrode and a second consumableelectrode movable in unison along a welding path between the edges oftwo adjacent, mutually grounded plates, a first power supply for passinga first welding current at a first low frequency between said firstelectrode and said plates, a second power supply for passing a secondwelding current at a second low frequency between said second electrodeand said plates, each of said power supplies including an inverter forconverting AC voltage to a DC current source having a maximum current ofat least 200 amperes with a positive terminal, a negative terminal, agrounded terminal and an output switching network including a firsttransistor based switch in series with said positive terminal, a firstinductor segment, one of said electrodes and said plates, a secondtransistor based switch in series with said negative terminal, a secondinductor segment, said one electrode and said plates and control meansfor alternately turning said first switch on and said second switch offat a first switch reversing point and turning said second switch on andsaid first switch off at a second switch reversing point to create an AChigh welding current with alternate positive and negative currentpulses.
 40. An electric arc welding apparatus as defined in claim 39wherein said first and second low frequencies are in the general rangeof 5 to 200 Hz.
 41. An electric arc welding apparatus as defined inclaim 39 wherein said AC voltage is three phase line voltage with afrequency of 10 or 60 Hz and said first and second low frequencies areindependent of said line voltage frequency.
 42. An electric arc weldingapparatus as defined in claim 41 wherein said means for adjusting saidgiven low frequency of at least one of said first and second powersupplies includes means for causing said given low frequency to vary asa function of time.
 43. An electric arc welding apparatus as defined inclaim 41 wherein both of said first and second power supplies includemeans for causing said given low frequency to vary as a function oftime.
 44. The electric arc welding apparatus as defined in claim 41wherein said low frequencies are less than about 300 Hz.
 45. An electricarc welding apparatus as defined in claim 39 wherein said means foradjusting said given low frequency of at least one of said first andsecond power supplies includes means for causing said given lowfrequency to vary as a function of time.
 46. The electric arc weldingapparatus as defined in claim 45 wherein said low frequencies are lessthan about 300 Hz.
 47. An electric arc welding apparatus as defined inclaim 39 wherein both of said first and second power supplies includemeans for causing said given low frequency to vary as a function oftime.
 48. The electric arc welding apparatus as defined in claim 47wherein said low frequencies are less than about 300 Hz.
 49. Theelectric arc welding apparatus as defined in claim 39 wherein said lowfrequencies are less than about 300 Hz.
 50. A method of electric arcwelding the joint between two plates, said method comprising: (a) movingfirst and second consumable electrodes in unison along said joint; (b)passing a first pulsating welding current between said first electrodeand said plates with a first low frequency; (c) passing a secondpulsating welding current between said second electrode and said plateswith a second low frequency; and, (d) varying at least one of said lowfrequencies as a function of time.
 51. The method as defined in claim 50wherein both of said low frequencies are varied as a function of time.52. The method as defined in claim 51 wherein said first and secondpulsating currents are AC currents.
 53. The method as defined in claim51 wherein said first and second pulsating currents are DC currents. 54.The method as defined in claim 51 wherein said first and second weldingcurrents are each created by a high frequency switch inverter driven bya three phase line voltage.
 55. The method as defined in claim 54wherein said high frequency is at least 20 kHz.
 56. A method as definedin claim 51 wherein said first and second welding currents areindependently created from the same three phase power supply.
 57. Themethod as defined in claim 50 wherein said first and second pulsatingcurrents are AC currents.
 58. The method as defined in claim 50 whereinsaid first and second pulsating currents are DC currents.
 59. The methodas defined in claim 50 wherein said first and second welding currentsare each created by a high frequency switch inverter driven by a threephase line voltage.
 60. The method as defined in claim 59 wherein saidhigh frequency is at least 20 kHz.
 61. A method as defined in claim 50wherein said first and second welding currents are independently createdfrom the same three phase power supply.
 62. An electric arc weldingapparatus comprising at least a first consumable electrode and a secondconsumable electrode movable in unison along a welding path between theedges of two adjacent, mutually grounded plates, a first power supplyfor passing a first welding current between said first electrode andsaid plates, a second power supplv for passing a second welding currentbetween said second electrode and said plates, each of said powersupplies including a three phase voltage input operated at linefrequency, a rectifier to convert said input voltage to a DC voltagelink and a high frequency switching type inverter converting said DCvoltage link to a high frequency AC current, an output rectifier circuitto provide a positive voltage terminal and a negative voltage terminal,and an output switching network for directing a welding current fromsaid terminals across one of said electrodes and said plates, and acircuit for independently adjusting said output switching network so thevalue of said first welding current of said first power supply isdifferent from said second welding current of said second power supply,wherein said output switching network of each power supply includes afirst switch to create positive current across one of said electrodesand said plate and a second switch to create a negative current acrossone of said electrodes and said plate and a circuit to operate saidfirst and second switches to control said welding current.
 63. Anelectric arc welder as defined in claim 62 wherein said circuit includesmeans for maintaining one of said switches closed and the other of saidswitches opened.
 64. An electric arc welder as defined in claim 62wherein said circuit includes means/or opening and closing said switchesat a rate to create an AC welding current housing a selected frequencyof 5-200 Hz.
 65. An electric arc welder as defined in claim 64 whereinsaid selected frequency is different for each of said first powersupply.
 66. An electric arc welding apparatus comprising at least afirst consumable electrode and a second consumable electrode movable inunison along a welding path between the edges of two adjacent, mutuallygrounded plates, a first power supply for passing a first low frequencywelding current between said first electrode and said plates, a secondpower supply for passing a second low frequency welding current betweensaid second electrode and said plates, each of said power suppliesincluding a three phase voltage input operated at line frequency, arectifier to convert said input voltage to a DC voltage link and a highfrequency switching type inverter converting said DC voltage link to ahigh frequency AC current, an output rectifier circuit to provide apositive voltage terminal and a negative voltage terminal, and an outputswitching network operated at a given low frequency for directing apulsating welding current at said given low frequency from saidterminals across one of said electrodes and said plates, a mastercontroller for creating a synchronizing signal alternating between apositive command and a negative command at a selected frequency, meansfor driving said first power supply by said synchronizing signal wherebysaid frequency of said first current is a function of said selectedfrequency, means for driving said second power supply by saidsynchronizing signal whereby said frequency of said second current is afunction of said selected frequency, means for delaying saidsynchronizing signal to said second power supply to phase shift saidsecond current from said first current, and means for operating one ofsaid power supplies at a frequency different from said selectedfrequency upon response to one of said commands from said synchronizingsignal.
 67. An electric arc welding apparatus as defined in claim 66wherein said operating means is an oscillator network having an outputoperated at said new frequency and driving said one of said powersupplies and means for restarting said operating means upon receipt ofsaid one of said commands from said synchronizing signal.
 68. Anelectric arc welding apparatus as defined in claim 66 wherein said firstand second low frequencies are in the general range of 5 to 200 Hz. 69.An electric arc welding apparatus as defined in claim 66 wherein saidconsumable electrodes are advance welding wires.
 70. An electric arcwelding apparatus as defined in claim 66 wherein said high frequency ofeach power supply inverter is over about 20 kHz.
 71. An electric arcwelding apparatus comprising at least a first consumable electrode and asecond consumable electrode movable in unison along a welding pathbetween the edges of two adjacent, mutually grounded plates, a firstpower supply for passing a first low frequency welding current exceeding200 amperes between said first electrode and said plates, a second powersupply for passing a second low frequency welding current exceeding 200amperes between said second electrode and said plates, each of saidpower supplies including a three phase voltage input operated at linefrequency, a rectifier to convert said input voltage to a DC voltagelink and a high frequency switching type inverter converting said DCvoltage link to a high frequency AC current, an output rectifier circuitto provide a positive voltage terminal and a negative voltage terminal,and an output switching network operated at a given low frequency fordirecting a pulsating welding current at said given low frequency fromsaid terminals across one of said electrodes and said plates, a mastercontroller for creating a synchronizing signal directed to said powersupplies and having a succession of synchronizing commands and means forforcing said power supplies to start its low frequency current uponreceipt of a synchronizing command.
 72. An electric arc weldingapparatus as defined in claim 71 including a delay circuit for delayingat one of said power supplies receipt of said synchronizing command fora selected time to phase shift said first and second welding currents.73. An electric arc welding apparatus as defined in claim 72 includingmeans for creating said low frequency of at least one of said powersupplies upon receipt of said synchronizing command.